On May 3, 2016, when Donald Trump appeared to clinch the Republican nomination by winning the Indiana primary, an elderly remnant of Hoosiers could remember the unlikely ascent of another businessman who, in 1940, came out of nowhere, convincing the G.O.P. that he, rather than someone from an array of familiar officeholders, should be the Party’s Presidential candidate. Wendell Willkie had so recently been a Democrat that he slipped up and called the convention delegates who nominated him “you Republicans.” His abrupt rise prompted an immortal complaint from one Party stalwart: “I don’t mind the church converting a whore, but I don’t like her to lead the choir the first night.”

Willkie did not become President, but during the next few years he did become a superior political specimen, an ever more reflective, out-of-the-box character whose place in history is not as Trump’s precursor but as his nontoxic opposite. Willkie rescued his new party from isolationism; Trump, seventy-six years later, converted its foreign policy into a supine, Russophilic reincarnation of America First doctrine. Willkie stayed on his dark horse after losing to Franklin Roosevelt, discovering that defeat, however painful, became him. Willing to engage in often exasperating coöperation with the President, he ended up proposing a bolder and more inclusive version of his former opponent’s plans for a postwar world. After his sudden death, in 1944, at the age of fifty-two, Willkie was recalled as a shining figure whom few ever regretted voting for. One biographer, Steve Neal, writing four decades after the failed run against F.D.R., asserted that “largely because of Willkie, Americans entered the war with a common purpose.” Now, three more decades on, David Levering Lewis, the Pulitzer Prize-winning biographer of W. E. B. Du Bois, presents “The Improbable Wendell Willkie” (Liveright), with a subtitle that does nothing by halves: “The Businessman Who Saved the Republican Party and His Country, and Conceived a New World Order.”

Wendell Willkie was born in Elwood, Indiana, on February 18, 1892. Lewis stresses his identity as a “Forty-Eighter descendant,” an heir to the wave of German liberals who fled Europe when the revolutions of 1848 came undone. As the Willkie (originally Willcke) family took root in Indiana, they earned a reputation for eccentricity and vanguardism. Wendell’s grandmother was a circuit-riding preacher; his mother was an attorney who practiced with her husband in Elwood. At the end of the eighteen-eighties, the town had been transformed by the natural-gas industry into a sort of vapored wonderland.“Geysers of ignited gas (‘flambeaux,’ Hoosiers called them) now, almost magically, lit up Elwood’s newly paved streets night and day,” Lewis writes. But the town remained vulnerable to general economic calamities, like the Panic of 1893, as well as to its own profligacy: in 1903, the gas ran out.

The Willkies boomed and busted with the rest of Elwood, and Herman Willkie, Wendell’s father, took a stand against “unfettered capitalism,” joining the economic-populist crusade of William Jennings Bryan. (Bryan spent a night at the Willkie home in 1900, during his second losing campaign against William McKinley.) Herman became the lawyer for the local tinplate workers’ union, and in the summer of 1909, alongside his brawny seventeen-year-old son, got into a bruising clash with strikebreakers. Three years later, Wendell, as a junior at Indiana University, petitioned for a course in socialism.

After graduation, Willkie spent six months in Puerto Rico, working as a chemist for one of the sugar trusts in order to save up for I.U.’s law school. The brutality he witnessed in the Caribbean—a plantation manager taking a machete to the shoulder of a rebellious worker—left him with a lasting revulsion toward the cruelties of imperialism. In 1916, he addressed his fellow law graduates on the need for an assortment of judicial and business reforms, and, after heading Elwood’s Young Democratic Club, he trained for service in the First World War. He reached France too late for combat but returned home a passionate supporter of Woodrow Wilson’s internationalist aims. ...